This invention relates to an optical waveguide having a substantially planar substrate and an inorganic waveguide layer applied to the substrate. The invention also relates to a process for the production of said optical waveguide and to the use of such waveguides.
The application of optical coatings to various substrates is known per se wherein, depending on the intended usage and manner of usage of the coated substrates, the optical quality of the coatings must meet different requirements.
A process for applying multiple-layer optical interference coatings to substrates having a complex surface configuration has been known from DOS 3,833,501. Thus, several hundred layers are applied to a polymer substrate by plasma-enhanced chemical vapor deposition wherein organometallic compounds are utilized for the coating. The coating method yields interference layers of a material with a relatively high organic proportion whereby high flexibility is ensured in the selection of the material for the coating step. The quality of the coating, especially with respect to the microstructure, is not of decisive importance for the purpose for which such substrates provided with several interference layers are employed, such as, for example, in helmet visors for helicopter pilots. It is true that mention is made of the fact that the coating does not exhibit the usually present micro-column structure, although this is demonstrated only for coatings of glass substrates. Besides, the lack of a micro-column structure can supposedly be traced back to the relatively high organic proportion of the coating. There is no suggestion in DOS 3,833,501 involving the production of optical waveguides with essentially planar substrates.
DOS 4,008,405.1 likewise lacks any hint with regard to optical waveguides with essentially planar substrates. Although the reference mentions coating synthetic resin substrates with interference layers to form reflectors, it is expressly pointed out that plastic substrates due to their low thermal stress-bearing capacity are less well suited than inorganic substrates, such as glass substrates, for example.
In contrast to reflectors or substrates generally equipped with interference layers, the requirements to be met by optical waveguides utilized, for example, as optical transducers in surface sensors, e.g. biosensors, are far higher. In particular, optical waveguides are desired which exhibit minimum attenuation and high refractive index. Such waveguides are known per se, but in each case glass substrates are utilized provided with inorganic or organic coatings. The low attenuation of these conventional waveguides is made possible by the fact that the inorganic substrate, such as glass, for example, can be heated to a high temperature so that the formation of a micro-column structure is extensively precluded.
An optical planar waveguide has been known from Kunz et al. in "Eurosensors" IV, 1990, Karlsruhe, which is produced with a reactive ion plating from Ta.sub.2 O.sub.5 and exhibits a refractive index of 2.2 at a loss of 1.1 dB/cm for the TE0 mode and 1.3 dB/cm for the TM0 mode. The effect of the substrate material on the losses at a wavelength of 633 nm is disclosed wherein layers of quartz glass have the lowest losses. When using this substrate, the losses are low (&lt;4 dB/cm) at low arc current, but the layer has a pronounced columnar structure rendering the layer dependent on environmental conditions. In order to obtain dense layers, the arc current must be brought to a high level, but this has the drawback that the losses are increased.
Lam, D. K. W., Appl. Opt. 23/1984/2744 discloses the production of an SiO.sub.x N.sub.y waveguide at a substrate temperature of 220.degree. C. on quartz glass or silicon. Low losses of SiO.sub.x N.sub.y are obtained only for a refractive index of about 1.75, namely after an additional process step compatible only for inorganic substrates. This involves an additional CO.sub.2 laser treatment during which the loss of 5 dB/cm is reduced to 1.5 dB/cm.
Application possibilities are described in Lukosz, W., et al., "Sensors and Actuators", B1 (1990): 585-588 and 592-596 for a waveguide produced by the sol-gel technique and having a thickness of about 170 nm, made up of an SiO.sub.2 --TiO.sub.2 mixed layer on glass with a refractive index of 1.74-1.80. The gel layer can be embossed so that a grating can be applied. However, after embossing the gel layer must be hardened with the disadvantage that the gel layer shrinks and thus its optical properties are altered. Pure TiO.sub.2 layers with a high index of refraction cannot be produced since the hardening-process requires temperatures so high that the layers become crystalline.
In Heuberger et al., Appl. Opt. 25/1986/1499, waveguides are described which are likewise produced by the sol-gel technique. The thus-applied SiO.sub.2 --TiO.sub.2 mixed layer on "Pyrex" glass has a loss of &lt;1 dB/cm, but must likewise be cured at high temperatures (500.degree. C.).
In all publications concerning optical waveguides, the production of SiO.sub.x N.sub.y, Ta.sub.2 O.sub.5 or SiO.sub.2 --TiO.sub.2 layers is described, in each case on inorganic substrates, wherein either high substrate temperatures must be employed, or an additional treatment must be performed.
A further disadvantage of the conventional waveguides resides in that their substrates proper cannot be embossed which, however, is necessary, for example, for the economical application of an optical grating for the coupling in or coupling out of light as regards the waveguide layer. Moreover, the known optical waveguides are fragile and cannot be processed by cutting or punching.
U.S. Pat. No. 4,749,245 discloses an optical waveguide on a planar substrate comprising an organic waveguide layer and a substrate of a synthetic resin. At least one intermediate layer of a further organic high-molecular material is required between the substrate and the waveguide layer; the solubility of this intermediate layer is different from that of the waveguide organic material and its refractive index is lower. One drawback of this waveguide resides in that only a restricted refractive index range is available for the waveguide layer (maximally up to 1.7), and that the waveguide layer is not stable with respect to its index of refraction under varying environmental conditions (moisture, temperature) since synthetic resins normally absorb moisture. Therefore, waveguides of the disclosed type are unsuitable for surface sensor technology [see, for example, R. Reuter et al., APL 52, p. 778 (1988)].